An independent study released by the state Department of Transportation Monday found a tunnel could replace the elevated portion of Interstate 81 in Syracuse, but it will cost significantly more than installing a boulevard or rebuilding the existing viaduct.

WSP, an internationally-renowned engineering consulting firm, examined seven potential tunnel options before narrowing the list to four of the most feasible projects. Depending on the project, the tunnel would range in length from 1.2 miles to 2.6 miles. The 1.2-mile tunnel would have the lowest price tag — $3 billion for a nine-year construction project. The 2.6-mile option would cost $4.5 billion and take 10 years to build.

Each of the tunnel alternatives considered by WSP would require street-level improvements, according to the study.

The firm concluded the best tunnel option was a $3.6 billion option — known as the "Orange Alternative" — that would take nine years to build. It would be positioned west of the current I-81 viaduct and maintain the ability to connect to Interstate 690 from I-81. To build the tunnel outlined in this proposal, it would require the acquisition of 22 properties in the city.

"We thank WSP for conducting this thorough study related to tunnel and depressed highway options, which will guide our decision-making process as plans for I-81 are considered," said Paul Karas, the state's acting transportation commissioner.

The study was ordered by Gov. Andrew Cuomo earlier this year. He wanted a review of five alternatives: A boulevard option; a tunnel; a depressed highway; a hybrid community grid-tunnel; and a depressed highway-community grid hybrid.

WSP did review options for a depressed highway to replace the I-81 viaduct, but found numerous challenges with each alternative. One problem was finding a way to establish a connection between I-81 as a depressed highway and I-690, which is not. The firm considered putting both highways underground, but determined having an underground interchange would pose construction issues and raise the price of the overall project.

The depressed highway alternatives were viewed as "detrimental to the city," according to WSP's report. They believe any of the options would divide neighborhoods and close off local streets. None of the depressed highway options were recommended to the state Department of Transportation.

WSP didn't examine options already reviewed by the department, including rebuilding the viaduct or removing the elevated highway and replacing it with a boulevard. The boulevard, known as the "community grid" option, would cost $1.3 billion. Rebuilding the viaduct would cost $1.7 billion.

Save81, a group of business and community leaders that supports maintaining I-81 in its current alignment, said in a statement Monday that it is "pleased" the independent analysis determined the feasibility of some tunnel alternatives.

"The continued operation of the I-81 corridor is vital to the economic stability of our region," the group said. "We look forward to reading the report when it is made available."

The state will make a decision on the project in the coming weeks. It's possible that the future of the I-81 viaduct will be revealed in early 2018, when Gov. Andrew Cuomo unveils his budget proposal and legislative agenda for the upcoming fiscal year.

Online producer Robert Harding can be reached at (315) 282-2220 or robert.harding@lee.net. Follow him on Twitter @robertharding.